these old trucks didn't have any pvc valves (or positive crankcase ventilation valve) they just have that pipe that comes out near the distributor and turns down and points toward the ground. newer cars have a pvc valve, usually mounted on the valve cover (at least i think so) and when the pressure gets to a certain point, it opens up and sends the fumes to the intake manifold via a solid or rubber vac line and the fumes get burnt with the new fuel instead being put into the air. that why its illegal to have breathers on newer model car because of environmental regulations.
http://en.wikipedia.org/wiki/PCV_valve
